Optical traffic preemption detector circuitry

Abstract

An optical traffic preemption detector detects pulses of light emitted by an approaching emergency vehicle and provides an output signal which is processed by a phase selector. The phase selector can request a traffic signal controller to preempt a normal traffic signal sequence to give priority to the emergency vehicle. A detector assembly is mounted in proximity to an intersection and can have multiple detector channels. A detector channel can have multiple photocells. Each photocell is provided with a rise time filter. If a detector channel has more than one photocell, the outputs of the respective rise time filters are coupled together. An output of a rise time filter, or coupled rise time filters, is first applied to a current-to-voltage converter and then a band pass filter. The band pass filter isolates a decaying sinusoid signal from a signal representative of a pulse of light. The decaying sinusoid signal is processed to produce a detector channel output signal that has a number of pulses for each pulse of light.

What is claimed is: 
     
       1. A detector for receiving pulses of light from an emergency vehicle and sending an output signal to a remote phase selector, the detector comprising: photocell means for providing an electrical signal in response to pulses of light received;   rise time filter means coupled to the photocell means for removing constant and slowly varying components from the electrical signal provided by the photocell means and allowing quickly changing pulse components of the electrical signal to pass;   band pass filter means coupled to the rise time filter means for generating a decaying packet of electrical pulses from each pulse provided by said rise time filter means; and   output means coupled to the band pass filter means for providing the output signal based upon the decaying packet pulses, whereby the amplitude of each received pulse of light is represented in said output signal as said packet of electrical pulses in which the number of successive pulses and amplitude of the final pulse in each packet represents the amplitude of the corresponding received pulse of light so that the output signal can have a relatively low maximum level consistent with integrated circuits and still be relatively immune to noise sources present in transmission lines coupling the detector to said remote phase selector.   
     
     
       2. The detector of claim 1 wherein the photocell means is a photodiode. 
     
     
       3. The detector of claim 2 wherein the photodiode operates in a photovoltaic mode and the electrical signal provided by the photocell means is a current signal that varies with the intensity of light striking the photocell means. 
     
     
       4. The detector of claim 3 wherein the rise time filter means comprises a capacitance and a resistance. 
     
     
       5. The detector of claim 4 wherein the photodiode has a first terminal and a second terminal, with the first terminal connected to ground, the resistance has a first terminal and a second terminal, with the first terminal connected to ground and the second terminal connected to the second terminal of the photodiode, and the capacitance has a first terminal and a second terminal, with the first terminal of the capacitance connected to the second terminal of the photodiode and the second terminal of the capacitance serving as an output of the rise time filter means. 
     
     
       6. The detector of claim 5 wherein the capacitance and the resistance form a high pass filter that removes from the current signal provided by the photocell means frequency components below approximately two kilohertz. 
     
     
       7. The detector of claim 1 wherein the band pass filter means has a center frequency. 
     
     
       8. The detector of claim 7 wherein the center frequency is approximately 6.5 kilohertz. 
     
     
       9. The detector of claim 1 wherein the band pass filter comprises first and second band pass filter stages. 
     
     
       10. The detector of claim 9 wherein each band pass filter stage comprises: an operational amplifier having an inverting input, a non-inverting input and an output, wherein the non-inverting input is connected to a bias voltage and the output also serves as an output for the band pass filter stage;   a first resistor connected between the output of the operational amplifier and the inverting input of the operational amplifier;   a second resistor connected between an input to the band pass filter stage and a common node;   a third resistor connected between the bias voltage and the common node;   a first capacitor connected between the output of the operational amplifier and the common node; and   a second capacitor connected between the inverting input of the operational amplifier and the common node.   
     
     
       11. The detector of claim 1 wherein the output means comprises: output power amplifier means, for providing an output signal capable of being received by a phase selector not in proximity to the detector.   
     
     
       12. The detector of claim 11 wherein the output means further comprises: shunting means for removing a negative component from the output signal of the output power amplifier means.   
     
     
       13. The detector of claim 11 wherein the output means further comprises: surge protection means for preventing the output signal of the output power amplifier means from exceeding limits imposed by a ground voltage and a supply voltage.   
     
     
       14. The detector of claim 11 wherein the output means further comprises: direct current blocking means, for removing a bias voltage from the output signal of the output power amplifier means.   
     
     
       15. The detector of claim 11 wherein the output power amplifier means comprises: an operational amplifier having an inverting input, a non-inverting input and an output, wherein the non-inverting input is connected to a bias voltage and the output also serves as an output for the output power amplifier means;   a first resistor connected between the output of the operational amplifier and the inverting input of the operational amplifier;   a first diode with an anode connected to the inverting input of the operational amplifier and a cathode connected to the output of the operational amplifier;   a second resistor connected between the inverting input of the operational amplifier and an input to the output power amplifier means.   
     
     
       16. The detector of claim 15 wherein the output power amplifier means further comprises: a second diode with an anode connected to the output of the operational amplifier and a cathode connected to a supply voltage; and   a third diode with an anode connected to a ground voltage and a cathode connected to the output of the operational amplifier.   
     
     
       17. The detector of claim 15 and further comprising: a second capacitor connected between the output of the output power amplifier means and the phase selector.   
     
     
       18. A detector channel for receiving pulses of light from an emergency vehicle and sending a signal to a remote phase selector, the detector comprising: first photocell means for providing an electrical signal that varies with an intensity of light striking the first photocell means;   first rise time means coupled to the first photocell means for removing constant and slowly varying components from the electrical signal provided by the first photocell means and allowing quickly changing pulse components of the electrical signal to pass;   summing means coupled to the first rise time filter means for combining an output from additional rise time filter means with an output from the first rise time filter means;   band pass filter means coupled to the summing means for generating a decaying packet of pulses from each pulse provided by the summing means; and   output means coupled to the band pass filter means for producing the output signal based upon the decaying packet pulses, whereby the amplitude of each received pulse of light is represented in said output signal as said packet of pulses in which the number of successive pulses and amplitude of the final pulse in each packet represents the amplitude of the corresponding received pulse of light so that the output signal can have a relatively low maximum level consistent with integrated circuits and still be relatively immune to noise sources present in transmission lines coupling the detector to said remote phase selector.   
     
     
       19. The detector of claim 18 wherein the summing means comprises: a circuit node that receives current signals from rise time filter means and provides an output current signal that represents the sum of the received currents; and   current-to-voltage converter means, for receiving the output current signal of the circuit node and providing an output voltage signal representative of the output current signal of the circuit node.   
     
     
       20. The detector of claim 19 wherein the current-to-voltage converter means comprises: an operational amplifier having an inverting input, a non-inverting input and an output, wherein the inverting input serves as an input to the current to voltage converter means, the noninverting inverting input is connected to a bias voltage and the output serves as an output of the current to voltage converter means; and   a resistor connected between the output of the operational amplifier and the inverting input of the operational amplifier.   
     
     
       21. The detector of claim 18 and further comprising: second photocell means, for providing an electrical signal that varies with an intensity of light striking the second photocell means; and   second rise time filter means coupled to the second photocell means and the summing means, for removing constant and slowly varying components from the electrical signal provided by the second photocell means and allowing quickly changing pulse components of the electrical signal to pass.